RNA editing alters the predicted nuclear or organelle gene products from many organisms, ranging from protozoans to humans. This distinctive form of RNA processing gives organisms unique flexibility in producing messenger and structural RNAs and proteins from DNA genomes. For organelles of some plants and animals, RNA editing can be viewed as a correction mechanism for genes that otherwise could not be expressed or would encode aberrant products. RNA editing also sometimes serves to allow the production of proteins with diverse functions or properties from a single gene. In humans, transcripts of genes involved in lipid metabolism and nervous system function are among those that undergo base modifications. Fundamental aspects of base modification RNA editing will be uncovered by studies of C-to-U RNA editing in plastids of angiosperm plants. Unlike other organelles in which RNA editing occurs plastids can be transformed with deliberately altered editing sites to analyze the cis-acting elements necessary for catalysis and recognition of bases for modification. Mutant screens will identify nuclear or organelle genes which affect editing site recognition and editing efficiency. With information obtained from transformation analysis, substrate RNAs can be designed to identify components of the editing apparatus that associate with target transcripts. The proposed research should enhance understanding of other forms of RNA processing in the biological world, especially the mechanisms of tRNA, mRNA and rRNA modification and editing.